This invention relates to a multilayered magnetic film to be used for planar type magnetic elements such as planar inductors and planar transformers and also to a thin film magnetic element comprising such a multilayered magnetic film.
In recent years, the down-sizing trend of equipment has been ever more remarkable. However, to date, the power source contained in such equipment have failed to be down-sized proportionally so that consequently the relative space occupied by the power source in the equipment has been on the rise. The down-sizing of electronic equipment owes greatly to the use of LSIs for electric circuits. On the other hand, miniaturization and circuit integration of inductors, transformers and other indispensable magnetic components of the power source have not been successful as that of the components of the electronic equipment itself. This is a principal cause of the fact that the relative volume of the power source in electronic equipment has been raised.
To solve this problem, the use of planar type soft magnetic elements that combine planar coils and magnetic members has been proposed. Studies are under way to improve the performance of planar magnetic elements. Unlike conventional magnetic elements, a planar type magnetic element is characterized in that the magnetic core is not limited to an inner core and may comprise a magnetic thin film or a magnetic foil.
Generally, the magnetic permeability in the high frequency zone is produced by way of a process of rotational magnetization. Therefore, magnetic excitation along the magnetization hard axis is indispensable under the condition of uniform in-plane uniaxial anisotropy to realize an ideal process of rotational magnetization. Then, physical properties including magnetic permeability and coercive force along the magnetization hard axis are important. The high frequency complex magnetic permeability of a specimen represents a value that is related to various physical properties of the specimen in a complicated way, although the dispersion of internal stress and magnetic anisotropy has to be sufficiently low to realize an ideal and planar real part and a sufficiently small imaginary part for the frequency characteristics of the specimen.
Thin film magnetic elements such as thin film inductors can provide a large electric power and a large saturation current when the magnetic film shows an enhanced level of saturation magnetization. Therefore, a high level of saturation magnetization is advantageous for the magnetic film of such an element. It is believed that a soft magnetic film having a high saturation magnetization and an in-plane uniaxial magnetic anisotropy and showing a low loss for high frequency magnetic excitation above 1 MHz can suitably be used for a planar type thin film magnetic element.
While the physical properties of the magnetic film are important for the planar thin film magnetic element, the multilayered structure including the magnetic film and the other layers such as a coil layer, an insulation layer and other layers and the interfaces of the layers of the laminate are also important from the viewpoint of the characteristics of the thin film magnetic element.
The interface of the resin layer and the multilayered magnetic film layer is indispensable for a known thin film magnetic element because the resin layer is particularly effective for filling the space of a coil layer for insulating the upper surface of the coil and for producing a plane surface. Particularly, the coiled section is required to have a significant height in a planar type magnetic element that has to deal with a large electric power and a large electric current at a low loss. For example, a resin material which is preferably polyimide can advantageously be used for filling the space of the coil layer having a height greater than 10 .mu.m.
Various techniques have been proposed for preparing a multilayered magnetic film.
For example, Japanese Patent Application Laid-Open No. 2-56754 discloses a technique for preparing a photomagnetic disc comprising steps of forming a buffer layer and a base protection layer by sputtering and subsequently forming a magnetic film thereon. Japanese Patent Application Laid-Open No. 4-134469 discloses a multi layered magnetic film obtained by laying a third magnetic film layer having an easily magnetizable direction on first and second magnetic film layers having a vertical magnetic anisotropy. Japanese Patent Application Laid-Open No. 63-113836 discloses a photomagnetic recording medium obtained by forming a layer of silicon nitride and aluminum nitride on a transparent substrate, the surface of which layer is then subjected to high frequency etching and sputtering, and subsequently forming thereon a magnetic film. Japanese Patent Application Laid-Open No. 63-211138 discloses a multilayered magnetic film obtained by sequentially laying an adhesive dielectric layer of a material such as silicon nitride on a plastic substrate and a non-crystalline magnetic layer thereon to produce an improved power of adhering to the substrate.
FIG. 13 schematically illustrates the structure of a known multilayered magnetic film.
Referring to FIG. 13, the multilayered magnetic film comprises a substrate 101 and a resin layer 102, an insulation layer 104, a magnetic layer 105 and another insulation layer 106 sequentially deposited on the substrate 101 to produce the multilayered magnetic film.
However, as pointed out above, in a known thin film magnetic element comprising such a multilayered magnetic film, the resin layer is particularly vulnerable to tensile stress and can easily give rise to cracks if compared with a thin film of a metal or a simple oxide. In view of this fact, the inventor of the present invention has succeeded in preparing a multilayered crack-free film comprising an insulation layer and a magnetic layer as ordinary multilayered magnetic film by observing certain conditions for forming film layers on a resin underlayer.
However, when such a multilayered magnetic film is compared with a conventional multilayered magnetic film prepared on a hard and flat substrate such as a substrate of thermally oxidized silicon, the high frequency magnetic characteristics of the former magnetic film are degradable to make it difficult to realize a high performance thin film magnetic element that reflects the proper characteristics of the magnetic layer. Presumably, this problem is attributable to an increase in the local anisotropic dispersion as discussed above and the resin underlayer that is typically made of polyimide may be damaged in various ways when the first insulation layer is formed for the multilayered magnetic film. The use of an AlN layer is indispensable from the viewpoint of the adaptability to the magnetic layer and to various processing operations and hence there is a need for a technique that can effectively reduce possible damages to the resin layer, while maintaining the structure of conventional multilayered magnetic films.
Meanwhile, the degradation of the high frequency magnetic characteristics of magnetic film was not taken into consideration in the development of the above listed known techniques.
Thus, there is a strong demand for a multilayered magnetic film that is formed on a resin underlayer but shows excellent magnetic characteristics comparable to those that can be obtained by forming film layers on a flat and highly rigid substrate or a multilayered magnetic film that maintains the excellent structure of conventional multilayered magnetic films comprising an insulation layer and a magnetic layer and, at the same time, can reduce possible damages to the resin layer. There is also a demand for a thin film magnetic element comprising such a multilayered magnetic film.